Gas sensors can be used to measure the concentration of a target gas. In most gas sensors the target gas is oxidized or reduced an electrode which results in a measureable sensor current. Integrated gas sensors utilize gas sensitive layers disposed on a semiconductor substrate. Many commercial chemical gas sensors utilize gas-sensitive metal oxide (MOX) layers disposed on semiconductor material. Such sensors may be produced at comparably low costs and exhibit a high sensitivity. Among MOX materials tin oxide is frequently used in solid-state sensors.
Recently graphene is used as a gas-sensitive sensor material due to its unique electrical properties. The band structure of graphene makes it particularly sensitive to chemical doping. The withdrawal or donation of even a few electrons shifts the Fermi level significantly away from the Dirac point, and thus even a small change in the number of charge carriers has a significant effect on the resistance of a graphene layer. Apart from its band structure, graphene has many other properties that render it particularly suitable for applications in gas sensors. Single-layer graphene has every atom at the surface, has a high metallic conductivity, even when very few charge carriers are present. Furthermore, it has and few crystal defects, which leads to low Johnson noise. The low noise level in graphene devices means that very small changes in resistivity (i.e. small sensor responses) can be measured, leading to highly sensitive sensors. Graphene is also chemically very stable due to its strong bonds and lack of defects. The electric conductivity of graphene allows for direct measurement of resistance, and the robustness of graphene allows for layers, which only one atom thick, to be processed into gas sensors.
Other gas sensors utilize a layer of two-dimensional electron gas (2DEG), which are sensitive to the presence of specific gases. For example, the two-dimensional electron gas (2DEG) formed at the interface of AlGaN/GaN layers grown on silicon substrates may be used for the detection of nitrogen oxides (NOx). In the presence of humidity, the interaction of nitrogen oxide with an open gate area may reversibly changes the conductivity of the 2DEG.
As outlined above, the measureable effect in solid-state gas sensors is usually a change of the electric conductivity (or resistivity) of gas-sensitive layers. Recent research has shown that gas sensitive layers (or generally chemically sensitive layers) such as graphene layers may also be used to form Hall bars. The measureable transversal voltage (e.g. the Hall voltage) due to the Hall effect shows also significant sensitivity to the presence of specific atoms or molecules of gaseous or liquid fluids. It is thus an object of the present invention to provide a sensor which makes use of the Hall effect in chemically sensitive layers.